Today, I was checking out a customer who seemed quite friendly. As I finished, he reached slightly over the counter and I impulsively reached out and shook his hand. He gave me a dumbfounded look and said, "Can I have my change please?" FML

And that's when you laugh, apologise and say it's been a long day. I've had several salespeople forget to give me my change or, on one occasion, even the stuff I bought; it's completely understandable. And if the client has a shred of a sense of humour or empathy, it's not even that awkward.

Haa. I've been there OP. Usually customers are fine with it if you just apologise and joke that it has "just been one of those days." Turning a potentially awkward moment into a light laugh. Customer service skills! :-P